Apparatus and method for R-wave detection with dual dynamic sensitivities

ABSTRACT

An apparatus and method for delivering electrical shock therapy in order to treat atrial tachyarrhythmias such as fibrillation utilizes a dynamically varying threshold to detect R-waves and synchronously deliver a defibrillation shock.

FIELD OF THE INVENTION

This invention pertains to methods for treating atrial tachyarrhythmias.In particular, the invention relates to an apparatus and method fordelivering shock therapy to terminate atrial fibrillation.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidheart rate, typically expressed in units of beats per minute (bpm). Theycan occur in either chamber of the heart (i.e., ventricles or atria) orboth. Examples of tachyarrhythmias include sinus tachycardia,ventricular tachycardia, ventricular fibrillation (VF), atrialtachycardia, and atrial fibrillation (AF). Tachycardia is characterizedby a rapid rate, either due to an ectopic excitatory focus or abnormalexcitation by normal pacemaker tissue. Fibrillation occurs when thechamber depolarizes in a chaotic fashion with abnormal depolarizationwaveforms as reflected by an EKG.

Cardioversion (an electrical shock delivered to the heart synchronouslywith an intrinsic depolarization) and defibrillation (an electricalshock delivered without such synchronization) can be used to terminatemost tachyarrhythmias by depolarizing excitable myocardium, whichthereby prolongs refractoriness, interrupts reentrant circuits, anddischarges excitatory foci. As used herein, the term defibrillationshould be taken to mean an electrical shock delivered eithersynchronously or not in order to terminate a fibrillation. Implantablecardioverter/defibrillators (ICDs) provide this kind of therapy bydelivering a shock pulse to the heart when fibrillation is detected bythe device. An ICD is a computerized device containing a pulse generatorthat is usually implanted into the chest or abdominal wall. Electrodesconnected by leads to the ICD are placed on the heart, or passedtransvenously into the heart, to sense cardiac activity and to conductthe shock pulses from the pulse generator. ICDs can be designed to treateither atrial or ventricular tachyarrhythmias, or both, and may alsoincorporate cardiac pacing functionality.

The most dangerous tachyarrythmias are ventricular tachycardia andventricular fibrillation, and ICDs have most commonly been applied inthe treatment of those conditions. ICDs are also capable, however, ofdetecting atrial tachyarrhythmias, such as atrial fibrillation andatrial flutter, and delivering a shock pulse to the atria in order toterminate the arrhythmia. Although not immediately life-threatening, itis important to treat atrial fibrillation for several reasons. First,atrial fibrillation is associated with a loss of atrio-ventricularsynchrony which can be hemodynamically compromising and cause suchsymptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillationcan also predispose to strokes resulting from emboli forming in the leftatrium. Although drug therapy and/or in-hospital cardioversion areacceptable treatment modalities for atrial fibrillation, ICDs configuredto treat atrial fibrillation offer a number of advantages to certainpatients, including convenience and greater efficacy.

As aforesaid, an ICD terminates atrial fibrillation by delivering ashock pulse to electrodes disposed in or near the atria. The resultingdepolarization also spreads to the ventricles, however, and there is arisk that such an atrial shock pulse can actually induce ventricularfibrillation, a condition much worse than atrial fibrillation. To lessenthis risk, current ICDs delay delivering an atrial shock pulse until theintrinsic ventricular rhythm is below a specified maximum rate and thendeliver the shock synchronously with a sensed ventricular depolarization(i.e., an R-wave). That is, an R-R interval, which is the time between apresently sensed R-wave and the preceding R-wave, is measured. If theR-R interval is above a specified minimum value, the interval isconsidered shockable and the atrial defibrillation shock pulse isdelivered.

Currently available implantable cardiac rhythm management devices,including bradycardia and tachycardia pacemakers and cardiacdefibrillators, have sense amplifier circuits for amplifying andfiltering electrogram signals picked up by electrodes placed in or onthe heart and which are coupled by suitable leads to the implantablecardiac rhythm management device. In some devices, the signals emanatingfrom the sense amplifier are applied to one input of a comparatorcircuit whose other input is connected to a source of referencepotential. Only when an electrogram signal from the sense amplifierexceeds the reference potential threshold will it be treated as adetected cardiac depolarization event such as an R-wave or a P-wave. Thesource reference potential may thus be referred to as a sensingthreshold. Other devices implement the comparator function in softwaresuch that a digitized electrogram signal value is compared with areference value in order to detect the depolarization event.

When a sensing threshold is set to a constant value, malsensing ofcardiac depolarization events can occur due to a number of factors.First, cardiac depolarization events can have widely different peakamplitudes, depending on patient activity body position, drugs beingused, etc. Lead movement and noise may further affect the detection ofcardiac depolarization events. Noise sources may include environmentalnoise, such as 60 Hz power line noise, myopotentials from skeletalmuscle, motion artifacts, baseline wander and T-waves. When noise levelsin the electrocardiogram approach the sensing threshold, the likelihoodof oversensing increases (i.e., false detection of depolarizationevents). If the sensing threshold is increased too high in an attempt toovercome the effects of noise, on the other hand, the likelihood ofundersensing (i.e., failing to detect depolarization events) isincreased. Methods have therefore been developed to automatically adjustthe sensing thresholds of cardiac rhythm management devices inaccordance with sensed activity. Such methods, however, present specialproblems with respect to R-wave detection in atrial defibrillators.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for delivering atrialdefibrillation shocks synchronously with R-waves detected using adynamic threshold with two sensitivities. In order to lessen theprobability of inducing ventricular fibrillation, an atrialdefibrillation shock is delivered in synchrony with an R-wave thatoccurs after a specified shockable time interval has elapsed as measuredfrom the preceding R-wave. In accordance with the invention, ventricularelectrograms are sensed through a ventricular sensing channel, and aventricular depolarization event (R-wave) is detected when a sensedventricular electrogram value exceeds a threshold that dynamicallyvaries in accordance with measured peak amplitudes. In order to providean R-wave detector with a desirable higher sensitivity within a shortinterval after an R-wave is detected and with a higher specificity afterthe shockable time interval has elapsed, two additional thresholds areemployed to detect R-waves for purposes of delivering an atrialdefibrillation shock. A low shock threshold with a low value relative tothe dynamically varying threshold is employed for detection immediatelyafter an R-wave is detected and thereafter until the shockable timeinterval elapses. After the shockable time interval has elapsed, a highshock threshold with a high value relative to the dynamically varyingthreshold is used to detect a subsequent R-wave.

In an exemplary embodiment, a dynamically varying threshold for R-wavedetection is provided as a threshold function that is set to a specifiedpercentage of the peak value of each detected R-wave and then decays toa base value (e.g., decreases exponentially or linearly). Upon detectingan episode of atrial fibrillation or other atrial tachyarrhythmia, anatrial defibrillation shock is delivered synchronously when thedynamically varying threshold detects an event, the interval from thelast event detected by the low shock threshold to the event presentlydetected by the dynamically varying threshold or the low shock thresholdis greater than the shockable time interval, and the high shockthreshold also detects an event within a specified time interval fromthe event detected by the low shock threshold or the dynamically varyingthreshold. The low shock threshold and the high shock threshold may bespecified as percentages of a specified minimum value for thedynamically varying threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of an implantable cardioverter/defibrillator.

FIGS. 2A through 2D depict electrograms and exemplary threshold valuesfor detecting R-waves.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a method and apparatus for delivering atrialdefibrillation shock therapy. As used herein, atrial defibrillationshock therapy should be taken to mean shock therapy for treating anyatrial tachyarrhythmia, such as atrial flutter, as well as atrialfibrillation.

FIG. 1 is a system diagram of a microprocessor-based implantablecardioverter/defibrillator device for treating atrial tachyarrthmiasthat also incorporates a pacemaker functionality. In this embodiment, amicroprocessor and associated circuitry make up the controller of thedevice, enabling it to output pacing or shock pulses in response tosensed events and lapsed time intervals. The microprocessor 10communicates with a memory 12 via a bidirectional data bus. The memory12 typically comprises a ROM or RAM for program storage and a RAM fordata storage. The ICD has atrial sensing and pacing channels comprisingelectrode 34, lead 33, sensing amplifier 31, pulse generator 32, and anatrial channel interface 30 which communicates bidirectionally with aport of microprocessor 10. The ventricular sensing and pacing channelssimilarly comprise electrode 24, lead 23, sensing amplifier 21, pulsegenerator 22, and a ventricular channel interface 20. For each channel,the same lead and electrode are used for both sensing and pacing. Theelectrogram signals generated by the sensing amplifiers may either befed to a comparator that inputs a digital signal to the microprocessoror digitized by an analog-to-digital converter and then input to themicroprocessor. The gain of the amplifier in each sensing channel may beadjusted by the microprocessor via an automatic gain control input AGCin accordance with sense signal amplitudes and/or measured noise levels.The sensing channels are used to control pacing and for measuring heartrate in order to detect tachyarrythmias such as fibrillation. The ICDdetects an atrial tachyarrhythmia, for example, by measuring the atrialrate as well as possibly performing other processing on data receivedfrom the atrial sensing channel. A shock pulse generator 50 isinterfaced to the microprocessor for delivering shock pulses to theatrium via a pair of terminals 51 a and 51 b that are connected bydefibrillation leads to shock electrodes placed in proximity to regionsof the heart. The defibrillation leads have along their lengthelectrically conductive coils that act as electrodes for defibrillationstimuli. A similar shock pulse generator 60 and shock electrodes 61 aand 61 b are provided to deliver ventricular fibrillation therapy in theevent of an induced ventricular fibrillation from atrial shock pulses.The shock pulse generators 50 and 60 include a capacitor that is chargedfrom a battery with an inductive boost converter to deliver the shockpulse.

When ventricular fibrillation is detected, the ICD charges up thecapacitor to a predetermined value for delivering a shock pulse ofsufficient magnitude to convert the fibrillation (i.e., thedefibrillation threshold). The capacitor is then connected to the shockelectrodes disposed in the heart to deliver the shock pulse. Sinceventricular fibrillation is immediately life threatening, these stepsare performed in rapid sequence with the shock pulse delivered as soonas possible.

Atrial defibrillation shocks are also delivered by charging an energystorage capacitor once atrial fibrillation is detected. The delivery ofthe shock pulse in this situation, however, does not take placeimmediately. In order to avoid the possible induction of ventricularfibrillation, atrial defibrillation shocks should be deliveredsynchronously with a sensed R-wave and after a minimum pre-shock R-Rinterval. (The R-R interval is the time between the immediatelypreceding R-wave and the presently sensed R-wave, and an R-wave may beregarded as either a spontaneously occurring depolarization or aventricular pace.) This is done because the ventricle is especiallyvulnerable to induction of fibrillation by a depolarizing shockdelivered at a time too near the end of the preceding ventricularcontraction (i.e., close to the T wave on an EKG). Delivering the shocksynchronously with a sensed R-wave thus moves the shock away from thevulnerable period, but at a very rapid ventricular rhythm, theventricular beats may be so close together that even synchronouslydelivered shocks may induce ventricular fibrillation. Shocking shouldtherefore be delayed until the ventricular rhythm is slow enough tosafely deliver the defibrillation pulse as determined by measuring theR-R interval. An R-R interval that meets a specified safety criterionfor shocking is termed a shockable R-R interval. Before delivering anatrial shock, the ventricular rhythm is monitored by measuring the R-Rinterval associated with each sensed R-wave. If a sensed R-wave occursat an R-R interval longer than a specified minimum limit value, theinterval is considered shockable so that the sensed R-wave is safe toshock on. An atrial defibrillation shock is then delivered immediatelyso as to be practically synchronous with the sensed R-wave.

In order to reduce malsensing of R-waves, the sensitivity of theventricular sensing channel may be automatically adjusted. In the deviceof FIG. 1, an automatic gain control imputs are provided for the sensingamplifiers that enable the controller to adjust the sensitivity of thesensing channels by changing the level of the electrogram signalgenerated by the sensing amplifiers. This type of sensitivity adjustmentwould normally be performed slowly over a long time period. A more rapidsensitivity adjustment may be implemented by providing a dynamicallyvarying threshold for detecting R-waves such as shown in FIGS. 2Athrough 2D. Electrogram signals that exceed the dynamically varyingthreshold DVT are interpreted as R-waves. The threshold DVT is set to aspecified percentage (e.g., 75%) of the measured peak amplitude of eachdetected R-wave and then decays (e.g., exponentially or linearly) to abase value. In this manner, the sensitivity of the channel may beadjusted automatically on a beat-to-beat basis to reflect changes in themeasured signal level.

A problem with the dynamically varying threshold for R-wave detection asdescribed above, however, is that the sensitivity of the channel is highduring the interval shortly after an R-wave is detected and thensubsequently decreases to a low level. This is not desirable behaviorwhen R-waves are detected for the purpose of delivering an atrialdefibrillation shock because if an R-wave is missed before the shockableinterval after a detected R-wave elapses, there is a danger that theshock will be delivered in synchrony with a subsequently detected R-wavethat does not meet the minimum shockable interval requirement. This isillustrated in FIG. 2A where small a amplitude R-wave is missed by thedetector after a detected R-wave because the small R-wave does notexceed the dynamically varying threshold DVT. Also, during the timeafter the shockable time interval since the detected R-wave has passed,there is the possibility that an atrial shock pulse will be delivered insynchrony with a falsely detected R-wave, owing to the low value of thedynamically varying threshold. FIG. 2B illustrates this situation wherea noise spike NS is interpreted as an R-wave because it exceeds thedynamically varying threshold DVT. Even if the shockable intervalcriterion is met at the time of the noise spike, it may still beclinically desirable to deliver the shock in synchrony with an actualR-wave. It is thus desirable for an R-wave detector used to deliveratrial defibrillation shocks to exhibit a high sensitivity whendetecting R-waves for starting measurement of an R-R interval and toexhibit a high specificity when detecting R-waves for delivering asynchronous shock pulse. Prior devices have dealt with this problem byproviding two R-wave detection channels, such as described in U.S. Pat.No. 5,562,709, assigned to Guidant Corp. and hereby incorporated byreference.

As aforesaid, the problems with using the dynamically varying thresholdfor detecting R-waves in order to deliver atrial defibrillation shocksare that of falsely negative detection shortly after a detected R-wavewhen the dynamically varying threshold is high and of falsely positivedetection after the threshold has decayed to a low value. The presentinvention deals with this problem by providing two additional R-wavethresholds to be used for delivering atrial defibrillation shocks: a lowshock threshold and a high shock threshold. The low shock threshold hasa low value relative to the dynamically varying threshold and isemployed for event detection after an R-wave is detected and before theshockable time interval elapses. This results in high sensitivitydetection of depolarization events within a short interval after a firstR-wave has been detected to lessen the possibility of missing such anevent and shocking on a subsequently detected R-wave that did not occurafter the minimum shockable time interval. After the shockable timeinterval has elapsed, the high shock threshold with a high valuerelative to the dynamically varying threshold is used to detect asubsequent R-wave for purposes of delivering a synchronous atrialdefibrillation shock. This results in detection with the desired higherspecificity in order to lessen the possibility of delivering a shock insynchrony with a falsely detected R-wave.

In accordance with the invention, an atrial defibrillation shock is thusdelivered when three conditions are met: 1) the dynamically varyingthreshold detects an event, 2) the interval from the last event detectedby the low shock threshold to the event presently detected by the lowshock threshold or the dynamically varying threshold is greater than theshockable time interval, and 3) the high shock threshold also detects anevent within a specified time interval from the event detected by thelow shock threshold or the dynamically varying threshold. One means ofimplementing this scheme is to start a shockable interval timer when asensed ventricular electrogram exceeds a specified low shock thresholdvalue. An atrial defibrillation shock pulse is then deliveredsynchronously with a detected R-wave if the shockable interval timer hasreached or exceeded a specified minimum value and if the sensedventricular electrogram value exceeds both the dynamically varyingthreshold value and the specified high shock threshold value. The lowshock threshold and the high shock threshold may be defined with respectto the minimum value that the dynamically varying threshold is allowedto have. For example, the low and high shock thresholds may specified as200 and 500 percent, respectively, of the specified minimum value forthe dynamically varying threshold.

FIG. 2C illustrates an example in which a premature ventricularcontraction (PVC) is detected by the low shock threshold LST where itwould have been missed if only the dynamically varying threshold DVTwere used. FIG. 2D shows a noise spike NS occurring after the minimumshockable interval. The noise spike exceeds the dynamically varyingthreshold DVT but does not exceed the high shock threshold HST.Therefore, no atrial defibrillation pulse is delivered.

The invention as described above may be implemented either with discretehardware components or with software executed by a microprocessor. Itshould also be noted that the method would not disturb any algorithmsfor detecting ventricular tachyarrhythmias using the dynamically varyingthreshold. That is, when detecting R-waves for purposes other thandelivering atrial defibrillation shocks, only the dynamically varyingthreshold can be used without regard to the low and high shock thresholdvalues.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A method for delivering atrial defibrillationtherapy, comprising: detecting an episode of atrial fibrillation orother atrial tachyarrhythmia; detecting ventricular depolarizations(R-waves) with a ventricular sensing channel, wherein R-waves aredetected when a sensed ventricular electrogram value exceeds adynamically varying threshold value; detecting separate events in theventricular sensing channel with a specified low shock threshold valueand a specified high shock threshold value; and, delivering an atrialdefibrillation shock pulse synchronously with a detected R-wave when thedynamically varying threshold detects an event, the interval from thelast event detected by the low shock threshold to the event presentlydetected by the low shock threshold or the dynamically varying thresholdis greater than a specified minimum shockable time interval, and thehigh shock threshold also detects an event within a specified maximumtime interval from the event detected by the low shock threshold or thedynamically varying threshold.
 2. The method of claim 1 furthercomprising: starting a shockable interval timer when a sensedventricular electrogram value exceeds the low shock threshold value;and, delivering an atrial defibrillation shock pulse synchronously witha detected R-wave if the shockable interval timer has reached orexceeded a specified minimum value and if the sensed ventricularelectrogram value exceeds both the dynamically varying threshold valueand the high shock threshold value within a specified maximum timeinterval.
 3. The method of claim 1 wherein the dynamically varyingthreshold value is set to a specified percentage of the peak absolutevalue of each detected R-wave and then decays to a specified minimumthreshold.
 4. The method of claim 3 wherein the dynamically varyingthreshold value decays exponentially.
 5. The method of claim 1 whereinthe specified low and high threshold values are specified percentages ofa specified minimum value for the dynamically varying threshold.
 6. Themethod of claim 1 further comprising dynamically adjusting the gain ofthe ventricular sensing channel.
 7. The method of claim 1 wherein anepisode of atrial fibrillation or other tachyarrhythmia is detected bydetecting atrial depolarizations with an atrial sensing channel anddetermining an atrial rate therefrom.
 8. The method of claim 1 furthercomprising detecting a ventricular tachyarrhythmia when a ventricularrate exceeds a specified limit value, with the ventricular ratedetermined as the interval between R-waves detected when a sensedventricular electrogram value exceeds the dynamically varying thresholdvalue.
 9. An apparatus for delivering atrial defibrillation therapy,comprising: an atrial sensing channel for detecting atrialdepolarizations (P waves); and; a ventricular sensing channel fordetecting ventricular depolarizations (R-waves); a shock pulse generatorfor generating atrial defibrillation shock pulses; a controller forcontrolling the operation of the device, wherein the controller isprogrammed to detect R-waves when a sensed ventricular electrogram valueexceeds a dynamically varying threshold value and to detect separateevents in the ventricular sensing channel with a specified low shockthreshold value and a specified high shock threshold value; and, whereinthe controller is programmed to detect episodes of atrial fibrillationor other atrial tachyarrhythmias from detected P waves, and, upondetection of an atrial tachyarrythmia, to deliver an atrialdefibrillation shock pulse synchronously with a detected R-wave when thedynamically varying threshold detects an event, the interval from thelast event detected by the low shock threshold to the event presentlydetected by the low shock threshold or the dynamically varying thresholdis greater than a specified minimum shockable time interval, and thehigh shock threshold also detects an event within a specified maximumtime interval from the event detected by the low shock threshold or thedynamically varying threshold.
 10. The device of claim 9 wherein thecontroller is programmed to start a shockable interval timer when asensed ventricular electrogram value exceeds a specified low thresholdvalue, and to deliver an atrial defibrillation shock pulse synchronouslywith a detected R-wave if the shockable interval timer has reached orexceeded a specified minimum value and if the sensed ventricularelectrogram value exceeds both the dynamically varying threshold valueand the high shock threshold value within a specified maximum timeinterval.
 11. The device of claim 9 wherein the dynamically varyingthreshold value is set to a specified percentage of the peak absolutevalue of each detected R-wave and then decays to a specified minimumthreshold.
 12. The device of claim 11 wherein the dynamically varyingthreshold value decays exponentially.
 13. The device of claim 11 whereinthe specified low and high threshold values are specified percentages ofa specified minimum value for the dynamically varying threshold.
 14. Thedevice of claim 9 wherein the gain of a sense amplifier in the atrialsensing channel is dynamically adjusted.
 15. The device of claim 9wherein the controller is programmed to detect an episode of atrialfibrillation or other tachyarrhythmia by detecting atrialdepolarizations with an atrial sensing channel and determining an atrialrate therefrom.
 16. The device of claim 9 wherein the controller isfurther programmed to detect a ventricular tachyarrhythmia when aventricular rate exceeds a specified limit value, with the ventricularrate determined as the interval between R-waves detected when a sensedventricular electrogram value exceeds the dynamically varying thresholdvalue.